


It's Gettin' Hot in (S)Fell

by QuintessenceA



Category: Undertale (Video Game)
Genre: Alternate Universe - Swapfell (Undertale), Consensual Somnophilia, Consensual Underage Sex, Ecto-Genitalia (Undertale), F/M, First Time, Light Masochism, Puppypaint, Swapfell Papyrus (Undertale), Underage Drinking, Undertale Skeletons in Heat
Language: English
Status: Completed
Published: 2019-11-14
Updated: 2019-11-14
Packaged: 2021-01-30 09:15:56
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 3,554
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21425815
Author URL: https://archiveofourown.org/users/QuintessenceA/pseuds/QuintessenceA
Summary: Throwing skeletons into heat is my new favorite thing! Here we have a Slim Pov, when his friend Cash's 17 y/o daughter drops by with a little 'problem'. Slim's a good guy though, who wouldn't take advantage of young lady in trouble. The girl, on the other hand...This is a gift for the wonderful MsBigBoots, creator of this OC Graffiti! I've ignored some canon to make this pre-story, self indulgent bit of first heat smut so I hope you don't mind XDCash is gonna be pissed, mwahaha.Detailed content in end notes, as one does. Or tries.
Comments: 2
Kudos: 18





	It's Gettin' Hot in (S)Fell

**Author's Note:**

  * For [msbigboots](https://archiveofourown.org/users/msbigboots/gifts).

Slim was about halfway into a fifth of fire whiskey he'd nicked from AU01 when the lights flickered. He scowled at the ceiling. Visitors. Wonderful. He took another swig from the bottle before setting it on the ground and muting the TV.

A minute later there were light, hurried footsteps on the porch. He didn't recognize them, or the rushed knocking that followed. Groaning, Slim stood up and walked over, pulling on his jacket as he did so. If something was up he could handle it. He was comfortably buzzed, but not really drunk. 

"Who's there?" He asked the door.

"Slim?" Graffiti's voice called from the otherside. Slim smiled. Well, if he had to have company she was probably the only monster he didn't mind coming over.

"Sorry pal," he called with a grin. "This AU already has a Slim. Try again later."

Graffiti laughed, and her slightly exasperated tone made him feel squishy feelings. "Aww, come on Slim, it's me, Graffiti. Seriously though, I needed to get away for a bit."

There was an uncharacteristic tremor to the end. Suddenly concerned, he wasted no time in unlocking the door and pulling it open. There she was, luckily whole and not bleeding or anything, standing on his porch, wrapped head to foot in an over-large jacket, sweatpants and that fluffy pink hat with the kitty ears. Under it all she was sweating heavily.

"Graffiti? Are you ok?"

"Hey. Yeah, I am but, um… can I come in?"

Slim nodded, stepping aside to clear the way. She gave him a grateful smile and stepped inside. 

"What's going o-" He cut off as the scent of her hit him and he nearly slammed the door shut in his hurry to close and lock it. "Oh. Shit."

Flushing lilac she ducked her head and rushed to stand by the couch. "Is it-?" She swallowed. "Is it that obvious? Do you want me to go? I- I didn't know who else to turn to. But I'll go, if you want."

"Uh, n-nah, it's hardly noticable," he lied, feeling his own cheekbones flush. "You're always welcome here." 

Pulling up his hood, he tried not to look at her directly as he took a seat at the kitchen table. But he could still smell her. Why was she turning to him, anyway? What did that even mean? "What's with the getup? Heading to the north pole?"

She gave a crooked smile, rubbing idly at one arm. "No, I thought maybe I could sweat out my problem. You know, like a fever?"

Slim laughed, and stifled it with a hand when Graffiti glared at him. "Sorry, I'm not laughing at you, really," he said, "but I'm afraid it doesn't work like that. Why don't you sit down and get out of those warm things, yeah? You've got to be miserable."

"Oh, you have no idea," she said, dropping to the couch.

"I just might," he muttered to himself as she began to pull them off. He'd had a heat stuck in Hotland once. Now that had sucked.

"It's been a nightmare back home," she continued, showing more and more bone as she removed her layers. And then, lovely light purple ectoflesh as she removed more. It was rude to stare, he told himself. She probably hadn't even meant to summon it. 

"My dad can't stand to have me in the house and Stretch acts like he's going to choke on his cigarette every time he catches sight of me! And don't get me started on the rest of them!" She shimmied out of her sweatpants, and Slim couldn't stop staring at the way her hips wiggled. 

Stars, he wanted to touch her, to feel her move like that under his claws as he pinned her hips down beneath him…

Wait, what the hell was he thinking? Discreetly he smacked himself in the face. She was practically his niece! And Void be damned, she was only 17 besides! 

Once she was down to just a pair of shorts and a tank top she leaned back into the couch with a moan of satisfaction. Heat rushed to fill his pelvis, ignoring all of his reasonable concerns as he saw the body she'd made. From an ample chest to those round hips to her thighs that looked good enough to eat…

Slim needed a cold shower. Or three. Graffiti didn't help matters as she turned to smile at him with half-lidded bedroom eyes. "That's why I came here. I knew you'd give me somewhere safe and quiet to crash while this burns through."

Nobody called Slim safe. And his imagination was full of ways to ruin quiet. He wanted to make her loud. No, bad, focus. He stared at the tabletop instead. "So, I guess you know what it is, but not how to clear it, right?"

"Right," she agreed. "My dad, helpful as always, keeps ducking my questions. Stretch," she said the name with frustration, "when I did corner him only said, 'it's different for guys' and vanished."

Slim snorted. "Liar."

"I know, right?!" Graffiti sighed, and he heard her leaning back to sink into the couch. "You'll help me, right?"

Stars, yes, he'd help. She only had to say the word. Slim cleared his throat. Questions. He can do that. "Have, uh, have you had one before?"

"Well, not exactly," she said with a nervous laugh. "Health class gave us all the boring details, but, ha, this is my first actual experience with it. It started yesterday."

Fuck. And that was all his libido needed to know. He shifted uncomfortably as his newly summoned cock pressed against his jeans. Maybe he should send her away too. Didn't she know how dangerous it was to wander a Fellverse while in heat? His instincts screamed at him to claim this breeding female that so willingly marched right into his lair. What was she thinking?

"Hey, Slim?" She called, and he realized he was still staring blankly at the table. He risked a glance over at her. A mistake, as her lovely eyelights watched him. It was only projection that made him see a hungry, familiar want in her gaze. Clearly.

"Yeah?" He asked. 

"You've had… heats," she hesitated on the word, her face glowing bright with embarrassment but she held his gaze. "In the past, I mean, right? Are they bad?"

Well, not what he expected to talk about tonight, that was for damn sure. "I have, yeah," he answered carefully. "If you go it alone, it'll take, like, five days to clear. It starts getting bad on about the second night. It's no fun, but it won't kill you."

"Yeah, that's what the book had said too," she said with a sigh. "Though you mentioned clearing it? Doesn't it always take five days?"

"No, it, heh… with help it can clear in as little as a single day or maybe two." He hoped she wouldn't ask him to help. He hoped she would.

"Help?" She whispered.

He closed his eyes, groaning at how close to begging that sounded. He couldn't think of how hopeful she'd sounded either. "Oh, come on baby girl, you're killing me here. Sex. You know?"

"Oh," she said. Her face was practically glowing lilac again, but the appraising look she was giving him made him wonder just what was going on behind those pretty eyelights of hers. "The book didn't exactly mention that part. But that definitely explains some… offers I got back home."

That caught Slim by surprise. Other monsters had propositioned her? HER? Jealousy unlike any he'd felt before filled his Soul, making his eyes narrow and his teeth clench. Graffiti was HIS. In HIS lair. On HIS couch. How DARE they. He would KILL them-

"Slim? Are you okay over there?" Graffiti's voice cut through his mindless anger and he jumped like he'd just been startled awake. "You're grabbing that table a little hard, don't you think?"

She was right. He hadn't even noticed driving his claws into the tabletop. Shit, he had to get ahold of himself. Graffiti wasn't his, he couldn't think that way.

"Hey, so I'm pretty tired," he admitted, prying his claws free from the wood. It was sorta true. "I'm gonna go lie down."

"What, already?" She asked, and he could see her trying to discreetly inspect the marks he'd left. "Are you sure? I- I wanted…"

He paused. "Yeah? What's that?" Slim's mouth went dry as (ha) bone. He wanted to give her anything, everything. His body and Soul were hers for the taking if she'd only ask.

"I…" Graffiti fidgeted on the couch, her thighs rubbing together. "I wanted… to know where the remote is?"

Of course that was all she needed. What had he thought she'd ask? For him to bend her over the couch and clear the heat now? That was stupid. He was stupid. His dick was stupid.

Slim stood up, tugging his jacket down to cover himself better. He passed her to reach the far end of the couch, retrieving the remote from where it had fallen between the cushions. Handing it over, her fingertips brushed his own and Slim saw stars.

The heat and smell of her was already growing stronger, if he didn't get away (and right fucking now) he was certainly going to do something they'd both regret, and Slim would never forgive himself. He couldn't keep the husky growl from his voice as he said, "House is as good as yours, help yourself to whatever you need."

With that, he beat a hasty retreat up the stairs.

Four hours later and he was lying in bed, sweaty and tired under a simple sheet. Finally he'd gotten his magic to dispell but he was far, far from satisfied. He was a terrible host, just leaving her downstairs like that, but he just couldn't risk it! Stars only knew what he'd do tomorrow. 

Part of him hoped she'd just leave, to put him out of his misery. Slim would never send her away himself, not when she needed him. Dammit, he could keep her safe, even from himself, right? As much as he hoped she'd go, what he really hoped for was…

The door to his bedroom creaked open, and the smell of Graffiti and his abandoned whiskey washed over him.

"S-Slim?" She called into the darkness. "Are you awake?" 

Slim was in no position to talk about this so he just remained quiet. 

"Stars," she added quietly, her steps unsteady as she walked into his room, "I hope you're sleeping."

Wait- what?

Any further questions were forstalled by the feel of her warm hand on his leg. His magic seemed to think this was a great time to reassert itself, and he heard her gasp as she saw its glow from under the sheet. 

He bit back a moan as she stroked his leg, but kept his eyesockets shut. Growing bolder, she moved further up the bed, her hand trailing along with her.

Slim felt a moment of shame over the state of his bed. His sheets only ever got changed when his brother dropped by to do his laundry, but he couldn't remember the last time that had been. It hardly seemed to matter to Graffiti as she pulled it away from his bones.

Slim knew he was nothing to look at. Too long, too old, too scarred. But his dick throbbed at the embarrassment of being exposed, at his helplessness, as self inflicted as it was. Graffiti's fingers ghosted up along his bones softly, reverently, like he was something special. Up the length of his femur, her finger tips brushing along his illiac crest, his spine, his ribs… She cupped his face with one hand, her thumb running over his gold tooth, feeling the crack in his mandible. 

"Slim?" She whispered again. "Are you awake?"

Nope, no wakefulness here. He could play dead. Hell, he could play coma all night long if it meant she'd keep touching him. Stars, he was disgusting.

"Stars, you're gorgeous..." She whispered against him, countering his unspoken thought. He could smell the whiskey on her breath even more strongly as she bent forward. "Whatever I want…?" And she kissed him. 

She moaned into his fangs, pressing her body against his ribcage. He wasn't sure if she was wearing a shirt any more. 

"Fuuuck…" she breathed, breaking the kiss and crawling onto the bed with him. Kneeling by his side, her warm hands grabbed his arm, drawing his hand closer and placing it on her breast.

Definitely no shirt. It was agony to keep himself from reacting but Graffiti was doing a good job of moving his hand for him, rubbing his thumb claw over her nipple as she began to breathe heavily above him. She hesitated a moment, holding his hand against her, and then squeezed it. Hard.

The pressure caused his claws to sink firmly into her ectoflesh, making her gasp in pleasure. She pulled on his hand gently, letting the tips score lines down her chest that had to be painful. It drew a ragged moan from her throat. Slim must be dreaming. It was the only explanation.

She moved his claw to her other breast, repeating the action using only one hand and Slim nearly choked as a slick, wet noise announced the location of her other hand.

Fuck, fuck, fuck. He needed her to touch him. His dick hurt with how hard he was. He was going to stroke out. He was-

"Ohhh," Graffiti moaned above him, shifting to straddle his pelvis. "Stars, I'm sorry Slim, I need more."

The heat from between her legs was intense. Lowering herself, she hesitated as the tip of his cock slipped between her lips, just tasting the wetness inside. His body trembled as he fought the urge to just blindly thrust up and into that waiting heat.

For a moment, he felt a brush of her emotions against his Soul. The touch was fleeting, but full of desire, need… and a spark of fear.

Oh, babygirl…

She was a virgin. Stars, fucking Stars, he needed to stop this, he couldn't do this to her. All he had to do was open his eyes, and she would stop. Her first time should be special, not with someone like him-

"Hnngh!"

The sound was pulled out of him as she pressed herself down, making her decision while he was still struggling with his. She froze at the noise, pausing halfway down his length. Her cunt gripped him spastically, and he tried desperately not to come already with how good, how wet, how hot she was!

"S-Slim? O-oh, fuck, you're awake…?" The horror in her voice made him crack his eyes open to find her staring at his hand, where he'd been clutching the bedsheet for dear life. Her chest was scored with the marks from his claws.

"Graffiti," he breathed her name like a prayer. "Please."

Her eyelights shot to his, sockets going wide. "Shit! Fuck! Slim, I'm sorry, I'm so sorry-"

"Please," he repeated, interrupting her. "More?"

"M-more? You want me to keep going?" She sounded so hopeful.

Slim groaned. "Stars, yes. Please. You're killing me here babygirl, I need you so bad."

Slowly, she pushed herself down further and this time Slim didn't hold back his moan of satisfaction as he sank deeply into her. She moaned with him, her face screwing up in pleasure.

"Oh, fuck, that's good. Babygirl, you're so fucking sexy. Ah, fuck me, I'm seeing stars. How you doing?"

Graffiti panted above him, shyly tucking her skull against her arm as she braced herself against his chest with both hands. She nodded. "I'm good. Really good. It, ah, you feel incredible."

"Ah, look at you," he breathed reverently. He couldn't believe this was happening. "You took me like a champ, didn't you? Can I touch you?"

She nodded, and he released the bed, grabbing her hips. "Here, move with me," he said, shifting his hips a little and she brought her head down to press her teeth against his clavicle, muffling a moan.

She let his hands guide her, riding him slow and gentle. The last thing he wanted to do was hurt her. He looked at the marks on her chest, a feral need rising in his Soul. Okay, he amended, maybe not the last thing. But he certainly didn't want to scare her.

He rubbed her, caressed her, worshiped her. It was better she was with him, Slim decided. He'll treat her right, he can give her what she needs, what she wants. Boys her age wouldn't appreciate her like he does.

The orgasm rolled up on her, and he could feel it building in the way she clenched around him. "Slim, Slim, oh Slim," she cried out softly, her voice rising until it broke, "St-Stars, Sli-i-im!"

He continued to push up into her, carrying her through the the climax until she slumped against him. Then he stilled, holding her as she panted, ignoring the demanding way his dick still throbbed inside her.

"Oh, Stars Slim, that was good," she said, staring at him with hazy eyelights. "But you," she hesitated, blushing. "You didn't…"

He smiled, pressing a kiss to her temple. "Don't worry about me babygirl. Relax a second, I got you." She melted into his arms and for a moment he just held her. Soon, she began moving again.

"It's so good, but please, please Slim, I need more!" She said, rocking against him. Her eyes fell to his claws again. "A-and you don't have to be so gentle, if you want more. I mean, I want it. You're strong. I- I like it."

She wanted more? Fuck, he wanted more. With a possessive growl, he rolled them both over, pinning her to the bed just like he'd imagined earlier. She gasped in delight, pleasure making her eyelights flare. He looked at the marks HIS claws had left on her chest. Who knew she'd be this receptive to a little rough handling?

"Fuck, do you have any idea what you're doing to me?" Slim asked, squeezing her hips. His claws didn't break the skin, but the threat of it had her gasping. "How you make me want to fuck you through the mattress? How I want to hold you, squeeze your pretty body, bite your pretty neck until you're screaming my name?"

She shuddered, grabbing desperately at his ribs as she tried to pull him closer, her hips thrusting up to meet him. Growling he pressed himself close, licking a stripe up the side of her neck, her jaw, and making her shiver. "Are you ready for this, babygirl? I'm gonna make you mine."

"Oh, yes, Slim, please!" 

Perhaps a little too roughly, he pushed his entire length into her, hilting in one smooth motion. She moaned into his chest as he wrapped his arms around her, crushing her arms between them and squeezing tightly as he began to thrust more quickly. 

"Ah! Ah! Slim! Stars yes!" 

He wanted to drag this out, to make it last, but now that he was taking her hard and fast, like he wanted, his climax was bearing down on him like a wave.

She was so beautiful, flushed with pleasure, moaning his name, writhing beneath him… that he lost his goddamn mind.

He bit down on her neck as he came, thrusting into her as deeply as he could reach, his fangs piercing the soft bone of her neck.

"SLIM!" She screamed his name, her body clenching around him as he pushed her over the edge to join him in oblivion.

When his mind cleared again he looked down at her, only to pull back, shocked at what he'd done. Hairline cracks radiated around her cervical vertebrae, centered around the puncture marks his fangs had left behind. Graffiti was lax beneath him, eye sockets closed and jaw slightly parted.

"Graffiti? Babygirl?" Slim asked worriedly. "Are, are you okay?"

"Hmm?" She asked drowsily, her eyes sockets opening to reveal eyelights that were fuzzed into little heart shapes. She blinked and they return to normal, focusing on him and she smiled.

"Oh, I'm doing amazing. I think you knocked the heat right out of me. I can't wait to tell my dad." 

Slims life passed before his eyes, ending with Graffiti screaming his name. Well, least he's going out on a high note.

Graffiti began to laugh beside him. "I'm only kidding!" She assured him, snuggling deeper into his side with a contented sigh. "Don't worry, I wouldn't let him kill you. I've got my dad wrapped around my little finger."

Slim was less then convinced, but it was a little late to worry about spilled milk now. "Well, at least let me heal your neck anyway. No point in putting that theory to test if we don't have to."

"All right," she grumbled sleepily, "if you come down here to do it. I think you've earned a nap."

"As you command," he chuckled, pulling her into his arms. As he began to caress her neck with the tips of his claws, doing what he could to call healing magic to the wound, she sighed contentedly.

"Anything you want, babygirl," he whispered to her as she fell asleep in his arms.

**Author's Note:**

> So, content warnings, let's try this. Slim drinks, leaves the bottle out for 17 year old Graffiti, who drinks. Dubcon for a first heat and Slim pretending to sleep through sex, but gets caught awake. Slim screws a 17 year old, with a lot of conflicting emotions including guilt, jealousy, and possessive behavior.


End file.
